There are known in the patent literature various types of interlocking plastic constructional panels, such as used for roofing. Examples of such panels appear in the following U.S. Pat. Nos. 4,790,112; 1,913,342; 3,062,338; 3,191,724; 3,236,017; 3,186,525; 3,172,508; 3,182,769; 3,984,961; 4,644,724; and 3,385,182. The panels illustrated and described in the above-mentioned patents have a great variety of constructional features, each having a specific purpose. U.S. Pat. No. 4,790,112 is particularly relevant in that it describes a non-uniform thickness interlocking panel which permits screw connection to an external support. However, a cap member is required to cover the screws.